We're getting a pretty solid bunch of tags for linguistics concepts, but most SE sites also have tags for kinds of questions which are useful too, especially when we identify the trends.
One trend is "Do languages exist with such and such?" and I think it might be useful to gather them under a tag but I'm not sure what to name it.
language-features possibly, but maybe "feature" has a technical sense which might make it ambiguous or confusing sometimes. Are there some other possibilities?
We already have reference-request, so how about language-request? Maybe it isn't the most intuitive name, except by analogy with reference-request, but it describes exactly what these questions are seeking.
How about language-lists? Such questions usually ask for a list of languages that satisfy some criteria.
I, too, was under the impression that we had a lot of these questions, but now that I look through the questions it seems that even if they are there, a lot of them are asking more about how a specific feature works and are tagged for that feature or an area of study. To me that's more important than grouping by question format (for example, questions about phonology are more closely related than questions that happen to ask "are there other languages with x").
If we do go with grouping these, cross-linguistic seems like a good place to start looking for questions that might fit. Regarding possible names:
Resources like WALS do use the word features but more generally call these properties, specifically "structural (phonological, grammatical, lexical) properties". In EL&U we use the term request for some of the questions asking to help find a word or find whether a term for something exists. Obviously this isn't a feature-request because that sounds like a request for an addition to software, but maybe it's a request to find a language that has a certain property—a language-property-request, cross-linguistic-request, something like that.
